Cannot use custom attribute type

[ilancoulon]

Not sure if this is related to #1092

I am trying to add my own attributes to my optimizer. The issue is that it seems I cannot use my own types, and I don't understand why. To be clearer, I can use my own MOI.AbstractOptimizerAttribute, but when I try do define a MOI.set function, it seems the third argument must be a standard type (actually, from this list: Union{Nothing, DataType, Number, Enum, AbstractString, MOI.AnyAttribute, MOI.AbstractSet, Function, MOI.ModelLike}).

Here is the code triggering an error:

using JuMP
using MathOptInterface
const MOI = MathOptInterface

struct MyOwnAttributeType <: MOI.AbstractOptimizerAttribute end

struct MyOwnAttribute end

struct OwnOptimizer <: MOI.AbstractOptimizer end

MOI.supports(::OwnOptimizer, ::MyOwnAttributeType) = true

MOI.is_empty(::OwnOptimizer) = true

function MOI.set(model::OwnOptimizer, ::MyOwnAttributeType, attr::MyOwnAttribute)
    println(attr)
end

function fail_set_attribute()
    model = Model(OwnOptimizer)
    MOI.set(model, MyOwnAttributeType(), MyOwnAttribute())

end

When launching fail_set_attribute(), I get the following error:

julia> fail_set_attribute()
ERROR: MethodError: no method matching map_indices(::MathOptInterface.Utilities.var"#7#8"{MathOptInterface.Utilities.IndexMap}, ::MyOwnAttribute)
Closest candidates are:
  map_indices(::AbstractDict{T,T}, ::Any) where T<:Union{MathOptInterface.VariableIndex, MathOptInterface.ConstraintIndex} at C:\Users\ilanc\.julia\packages\MathOptInterface\bygN7\src\Utilities\functions.jl:73
  map_indices(::Function, ::Union{Nothing, AbstractString, DataType, Function, Number, Tuple{Vararg{Union{Nothing, AbstractString, DataType, Function, Number, MathOptInterface.AbstractConstraintAttribute, MathOptInterface.AbstractModelAttribute, MathOptInterface.AbstractOptimizerAttribute, MathOptInterface.AbstractSet, MathOptInterface.AbstractVariableAttribute, MathOptInterface.ModelLike, Enum},N} where N}, MathOptInterface.AbstractConstraintAttribute, MathOptInterface.AbstractModelAttribute, MathOptInterface.AbstractOptimizerAttribute, MathOptInterface.AbstractSet, MathOptInterface.AbstractVariableAttribute, MathOptInterface.ModelLike, AbstractArray{#s61,N} where N where #s61<:Union{Nothing, AbstractString, DataType, Function, Number, Tuple{Vararg{Union{Nothing, AbstractString, DataType, Function, Number, MathOptInterface.AbstractConstraintAttribute, MathOptInterface.AbstractModelAttribute, MathOptInterface.AbstractOptimizerAttribute, MathOptInterface.AbstractSet, MathOptInterface.AbstractVariableAttribute, MathOptInterface.ModelLike, Enum},N} where N}, MathOptInterface.AbstractConstraintAttribute, MathOptInterface.AbstractModelAttribute, MathOptInterface.AbstractOptimizerAttribute, MathOptInterface.AbstractSet, MathOptInterface.AbstractVariableAttribute, MathOptInterface.ModelLike, Enum}, AbstractArray{#s60,N} where N where #s60<:(AbstractArray{#s14,N} where N where #s14<:Union{Nothing, AbstractString, DataType, Function, Number, Tuple{Vararg{Union{Nothing, AbstractString, DataType, Function, Number, MathOptInterface.AbstractConstraintAttribute, MathOptInterface.AbstractModelAttribute, MathOptInterface.AbstractOptimizerAttribute, MathOptInterface.AbstractSet, MathOptInterface.AbstractVariableAttribute, MathOptInterface.ModelLike, Enum},N} where N}, MathOptInterface.AbstractConstraintAttribute, MathOptInterface.AbstractModelAttribute, MathOptInterface.AbstractOptimizerAttribute, MathOptInterface.AbstractSet, MathOptInterface.AbstractVariableAttribute, MathOptInterface.ModelLike, Enum}), Enum}) at C:\Users\ilanc\.julia\packages\MathOptInterface\bygN7\src\Utilities\functions.jl:80
  map_indices(::Function, ::MathOptInterface.VariableIndex) at C:\Users\ilanc\.julia\packages\MathOptInterface\bygN7\src\Utilities\functions.jl:82
  ...
Stacktrace:
 [1] map_indices(::MathOptInterface.Utilities.IndexMap, ::MyOwnAttribute) at C:\Users\ilanc\.julia\packages\MathOptInterface\bygN7\src\Utilities\functions.jl:73
 [2] set(::MathOptInterface.Utilities.CachingOptimizer{MathOptInterface.AbstractOptimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}, ::MyOwnAttributeType, ::MyOwnAttribute) at C:\Users\ilanc\.julia\packages\MathOptInterface\bygN7\src\Utilities\cachingoptimizer.jl:626
 [3] set(::Model, ::MyOwnAttributeType, ::MyOwnAttribute) at C:\Users\ilanc\.julia\packages\JuMP\YXK4e\src\JuMP.jl:892
 [4] fail_set_attribute() at C:\Users\ilanc\Dropbox\Stage-MTL\julia\jump_attribute_type.jl:21
 [5] top-level scope at REPL[3]:1
 [6] eval(::Module, ::Any) at .\boot.jl:331
 [7] eval_user_input(::Any, ::REPL.REPLBackend) at D:\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.4\REPL\src\REPL.jl:86
 [8] run_backend(::REPL.REPLBackend) at C:\Users\ilanc\.julia\packages\Revise\BqeJF\src\Revise.jl:1184
 [9] top-level scope at REPL[1]:0

It would be pretty useful to have my own type as an attribute, do you think it is possible?

[odow]

Looks like you need to define map_indices:

julia> MOI.Utilities.map_indices(::Function, x::MyOwnAttribute) = x

julia> fail_set_attribute();
MyOwnAttribute()

This should probably be documented, or we should loosen the fallback.

[blegat]

For custom values of attributes, you need to implement:
http://jump.dev/MathOptInterface.jl/stable/apireference/#MathOptInterface.Utilities.map_indices
if you want it to work with copy_to/CachingOptimizer and
http://jump.dev/MathOptInterface.jl/stable/apireference/#MathOptInterface.Utilities.substitute_variables
if you want it to work when there are bridged variables.
It's mentioned in the docstrings of the above methods but if you have an idea of doc improvement for this, it's welcome.

[ilancoulon]

Thank you very much!
I think it could be useful to add it there: https://jump.dev/MathOptInterface.jl/stable/apimanual/#Solver-specific-attributes-1
I don't really have time right now but I'll try to add it in a PR soon.